272 MODERN PAPER-MAKING
The wires used for making newsprint are almost invariably about 60 meshes
per inch in the cross direction and 40 in the machine direction, and the recently
developed long crimp or twill wires (see p. 166) are used very extensively
for news.
Drainage on the wire increases the consistency of stock from about 0.8 to
about 3 per cent, at which point the suction boxes or flat boxes, as they are
sometimes called, come into action, raising the consistency to around 16 per
cent. Finally the remaining water, which is capable of being removed while
the sheer is still in contact with the wire, is taken out by the couch where the
concentration is increased to about 20 per cent.
The finally diluted stock fed to the flow box of a newsprint machine is
very heterogeneous. The freeness will be about 50 c.c. Canadian standard,
and the china clay will account for about a quarter of the 0.8 per cent of total
solids, if an average ash content of about 8 per cent is desired in the sheet
being made. The quantity of very fine fibre and clay present in flow-box
stock is very much higher than it is in the thick stock delivered to the
mixing pump. This stock will have a Canadian freeness figure of about
125 c.c., and the total solids will have a china-clay content of about 10 per
cent.
The reason for the change in the composition and properties of the stock
when it gets to the flow box is that an amazingly large proportion of the stock
that flows on to the wire passes through the meshes by drainage and by the
action of the table rolls. Flow-box stock having a consistency of 0.8 per cent
will give rise to an average consistency in the wire pit of about 0.3 per cent.
This means that nearly 40 per cent goes through the wire. Most of this fine
short-fibred stock goes back again to the mixing pump to be delivered once
more to the flow box. It therefore does not represent lost material, but its
circulation back into the flow box is the reason why the stock issuing from the
slice is so very different in properties and composition from the thick stock
fed to the mixing pump.
The number of suction boxes fitted to the wire will usually be about eight,
and the suction will be graded from about 5 inches of mercury to about 10
or 12 inches. The tops of the boxes must be fitted with some material that
will give rise to as little friction as possible between the rapidly travelling wire
and the suction boxes. Many different materials, including glass, granite and
synthetic resins, have been tried, but by far the most popular is still wood.
A hard wood, such as maple, arranged so that the grain is perpendicular to
die wire, gives very satisfactory results, both because it requires attention less
frequently than horizontal-grain suction boxes, and also because the load on'
the couch motor is reduced to a mimmurn (see also p. 183).